The present invention relates to a reactive diluent for ultraviolet and electron beam curable resins.
In recent years, there has been actively forwarded a development of solventless resin compositions curable by radiation of ultraviolet rays or electron beams in view of environmental protection, labor saving and energy saving. In general, these UV or EB radiation curable resin compositions are composed mainly of a photopolymerizable prepolymer and a photopolymerizable vinyl monomer copolymerizable with the prepolymer. The UV curable resin compositions further contain a photoinitiator as an essential component.
The photopolymerizable prepolymers can further polymerize by radiation of ultraviolet rays or electron beams and form a framework of cured polymers. The prepolymer is also called a photopolymerizable unsaturated polymer or a photopolymerizable oligomer. In accordance with the structure of molecule constituting the framework, there are used, for instance, oligomers having acryloyl or methacryloyl group at the polymer end, such as polyester acrylate, polyurethane acrylate, epoxy acrylate, polyether acrylate, oligoacrylate, alkyd acrylate and polyol acrylate.
The photopolymerizable vinyl monomers serve, in cooperation with the prepolymer, to improve the functions of curable resin compositions or the applicability or suitability to uses of cured products. For instance, they are used for the purpose of decreasing the viscosity of the composition to thereby improve the coatability to substrates, imparting the flexibility to cured products, or improving the adhesion to substrates. Also, the curable composition itself can be made solventless by utilizing the dissolving power and polymerizability of the monomers. Since the photopolymerizable monomers are required to have a good copolymerizability with prepolymers and a high curing rate, polyfunctional or monofunctional acrylic monomers are usually used.
Monofunctional monomers serve as a diluent for highly viscous prepolymers, thereby securing the workability of resin compositions in practical use. Moreover, the monomers themselves polymerize to form a part of the structure of cured products without going out from the system. Therefore, it is possible to formulate the curable resin compositions into solventless compositions, thus the monomers have an important meaning in providing UV or EB radiation curable resin compositions free from solvent pollution. In this sense, the photopolymerizable monomers are also called a reactive diluent.
The reactive diluents are essential to have a large ability of lowering the viscosity of prepolymers from the viewpoint of the object of use. Also, it is desirable that the reactive diluents do not give out a bad smell or do not cause health disturbances such as dermatitis in handling them.
In general, however, acrylic monomers have a strong skin irritation and the improvement has been demanded. It is disclosed in Japanese Unexamined Patent Publication (Tokkyo Kokai) No. 147024/1978 that the skin irritation of acrylic monomers is decreased by increasing the molecular weight of monomers. However, when the molecular weight is increased too large, the ability of lowering the viscosity of prepolymers is decreased, because the viscosity of the monomers themselves rises. Also, in cases, the compatibility with prepolymers is decreased. For such reasons, there has been strongly desired a development of a reactive diluent monomer having a high dissolving power, a high diluting effect, a low volatility, a low smelling property, a low skin irritation, and moreover having an excellent curing activity.